@Joe: ...and not quite serious! (I've aready bought my 7-year old more lego than I've ever owned myself) At the same time, I don't let him play with my vintage / collectors items. He just isn't ready.
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KramiiOct 30 '11 at 20:25

"Don't play with lego on the floor where it can be stepped on". That's more for your own safety. Walking barefoot on small lego pieces hurts. A lot.
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Stephane DelcroixDec 24 '13 at 6:58

If I wouldn't have played with LEGO when I was a kid most likely I would be an engineering PhD student in one big university of the UK. LEGO are the ones that brought me here, so Keep LEGO close to children
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Ander BiguriMar 30 at 9:48

What, in your experience, causes them to break most easily?
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KramiiDec 7 '11 at 13:38

1

I think there were two reasons. When used in building it was used to attach pieces which ended up being horizontal, so if there wasn't enough support underneath you could flex it at the corner of the "L" and break it. The second reason I believe is that I had all bricks in a big bucket, I think the act of sifting through the pieces may have put stress on the piece enough to bend it and it would break. If I had compartmentalized storage that may have saved some pieces.
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ghoppeDec 7 '11 at 13:47

I too had a few of these pieces break, but not many other pieces. This is only wild speculation on my part, but it may be to do with a combination of two things: Design - Mine cracked horizontally between the studs across the hole, which is probably the weakest point due to the least amount of plastic. Sun exposure: My classic Space sets that used these pieces have quite a lot of fading and maybe the UV exposure made them brittle.
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mcqwertyJul 20 '12 at 18:30

The ones that gave me the most trouble were the "new-style" BIONICLE axle connectors like this one:

It seems that the parts holding the axle ball were much thinner than on the previous versions, and they tended to crack and break from extensive use. It was possible to use some cracked ones, but some broke beyond that.

I remember that back in the day (which would be mid eighties), I had a number of 1x1 translucent plates split on me. Right through the middle – and then I had two halves. They would typically split either when trying to connect or disconnect them to/from a regular stud. No idea if it was a bad batch or what. Haven't had that happen with more recent versions.

This weekend I sorted through some boxes of lego that have laid dormant under my parents' house since I moved out. They contained bricks from the 80s to late 90s. THe only broken pieces I found where these technic chain pieces, crushed at the bottom of the box, and lego traffic signs broken off at the base. I can reuse the traffic signs by sticking them into a 1x1x1 barrel piece, but the chain is ruined and I'll need to buy a whole new one.